Biventricular cardiac pacemakers are typically designed for stimulation of the right and left ventricles of the heart to perform cardiac resynchronization therapy (CRT), for example. To do so, a biventricular cardiac stimulator typically has a right-ventricular stimulation unit and a left-ventricular stimulation unit, each being connected or connectable to at least one right-ventricular and/or at least one left-ventricular stimulation electrode.
The right-ventricular sensing electrode(s) and the right-ventricular stimulation electrode(s) are typically attached to a right-ventricular electrode line, while the left-ventricular sensing electrode(s) and the left-ventricular stimulation electrode(s) are components of a left-ventricular electrode line. Such left-ventricular electrode lines are typically advanced through the coronary sinus into proximity to the left ventricle and therefore are also referred to as CS electrode lines. Left-ventricular electrode lines, in comparison with right-ventricular electrode lines, incur an increased probability that the electrode line will be shifted, which changes the sensing and stimulation conditions.
On the basis of stimulated or sensed events, a pacemaker timer determines points in time at which stimulation pulses are to be delivered to the respective chamber of the heart. A stimulated event is delivery of a stimulation pulse, which leads to a contraction of the respective chamber of the heart. A detected event, also known as a natural or intrinsic event, is an independent contraction of the corresponding chamber of the heart, which is detected by a corresponding sensing electrode. The corresponding sensing electrode detects the electric potentials associated with a natural contraction of a corresponding chamber of the heart. These potentials are amplified and evaluated by the pacemaker controller and, in particular, by the pacemaker timer.
As is known to those skilled in the art, a natural contraction of the right atrium can be detected as a so-called P wave portion of an electrocardiogram (ECG) signal. The corresponding intrinsic right-atrial event is referred to here as As (atrium sense). The natural contraction of the right ventricle is manifested in the form of an R wave in an is electrocardiogram. A detected natural contraction of the right ventricle is referred to here as RVs. A detected natural contraction of the left ventricle is referred to here as LVs. RVp denotes a stimulated, or paced, event in the right ventricle. Accordingly, LVp denotes a stimulated, or paced, event in the left ventricle. It is known that cardiac stimulators generate a marker signal to identify the respective detected (natural) event or stimulated event, characterizing the point in time corresponding to the respective event. The pacemaker timer can then access these marker signals. Marker signals are frequently transmitted in a respective channel, e.g., a right-atrial detection channel, a right-ventricular detection channel or a left-ventricular detection channel.
In this context, it is known that the points in time for delivery of a respective stimulation pulse may be determined, but the delivery of a stimulation pulse is to be suppressed (to be inhibited) if an independent contraction of the respective chamber of the heart, (i.e., an intrinsic event of this chamber of the heart) is detected within a certain interval before the intended stimulation time. If the cardiac stimulator is designed in this way, it stimulates the respective chamber of the heart only on demand and therefore the corresponding mode of operation is also known as demand mode.
As already mentioned, the points in time at which the respective next stimulation pulse of a respective chamber is provided are determined by the pacemaker timer on the basis of sensed events or stimulated events. In a known manner, the pacemaker timer is designed so that the chambers of the heart contract in a chronological order as closely as possible to the action of a healthy heart as a function of the respective hemodynamic demand. For example, a contraction of the right atrium after an atrioventricular conduction time is followed by a contraction of the right ventricle. In a similar manner, the pacemaker timer determines the point in time for delivery of the next right-ventricular stimulation pulse on the basis of an atrioventricular delay time (AVD), which is triggered by a stimulated or detected event in the right atrium. The atrioventricular delay time is advantageously variable, such that the pacemaker timer is able to adapt optimally to the respective hemodynamic demand and to the individual needs of the respective patient.
In a cardiac pacemaker which stimulates both the right atrium and the right ventricle, the pacemaker timer also determines the point in time of the next atrial stimulation after a VA delay time, which follows the respective stimulated or detected ventricular event and depends is to a substantial extent on the heart rate, which is adapted as well as possible to the hemodynamic demand. A cardiac stimulator having a pacemaker timer which can adapt the stimulation rate to the hemodynamic demand is known as a rate-adaptive cardiac stimulator.
In biventricular cardiac stimulators, the left ventricle may additionally be stimulated to synchronize the actions (contractions) of the right ventricle and the left ventricle as part of a cardiac resynchronization therapy (CRT). In this context, an interventricular delay time (VVD) also plays a role for the pacemaker timer, because the VVD describes the time delay between the intended delivery of a right-ventricular stimulation pulse and the intended delivery of a left-ventricular stimulation pulse. The VVD may be zero or even negative so that, for example, delivery of a left-ventricular stimulation pulse prior to delivery of a right-ventricular stimulation pulse may be provided. This interventricular delay time is preferably variable, in the sense that the pacemaker timer can adjust to individual needs and instantaneous requirements of a respective patient.
Because it is known in the art that some biventricular cardiac stimulators are capable of also stimulating the right atrium, and therefore they are considered to be “triple-chamber stimulators,” no further detailed description thereof need be given here.